refactor(header): Implement HttpDate, a wrapper for dates

Using `time::Tm` directly in HTTP header fields requires special handling to parse and format
the header values., this stops us from using the header macros. By wrapping `time::Time` in a
`HttpDate`, we can use the `FromStr` and `Display` traits of `HttpDate` like for most other values.

BREAKING_CHANGE: All code using one of the `Date`, `Expires`, `If-Modified-Since`,
`If-Unmodified-Since`, `Last-Modified` header fields needs to wrap `time::Tm`
with `HttpDate`. Removed `FromStr` trait of `Date`, `If-Modified-Sice` and `If-Unmodified-Sice`,
implementing the trait here is inconsistent with other headers.

src/header/shared/httpdate.rs

@@ -0,0 +1,91 @@
+use std::str::FromStr;
+use std::fmt::{self, Display};
+
+use time;
+
+/// A `time::Time` with HTTP formatting and parsing
+///
+//   Prior to 1995, there were three different formats commonly used by
+//   servers to communicate timestamps.  For compatibility with old
+//   implementations, all three are defined here.  The preferred format is
+//   a fixed-length and single-zone subset of the date and time
+//   specification used by the Internet Message Format [RFC5322].
+//
+//     HTTP-date    = IMF-fixdate / obs-date
+//
+//   An example of the preferred format is
+//
+//     Sun, 06 Nov 1994 08:49:37 GMT    ; IMF-fixdate
+//
+//   Examples of the two obsolete formats are
+//
+//     Sunday, 06-Nov-94 08:49:37 GMT   ; obsolete RFC 850 format
+//     Sun Nov  6 08:49:37 1994         ; ANSI C's asctime() format
+//
+//   A recipient that parses a timestamp value in an HTTP header field
+//   MUST accept all three HTTP-date formats.  When a sender generates a
+//   header field that contains one or more timestamps defined as
+//   HTTP-date, the sender MUST generate those timestamps in the
+//   IMF-fixdate format.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct HttpDate(pub time::Tm);
+
+impl FromStr for HttpDate {
+    type Err = ();
+    fn from_str(s: &str) -> Result<Self, ()> {
+        match time::strptime(s, "%a, %d %b %Y %T %Z").or_else(|_| {
+            time::strptime(s, "%A, %d-%b-%y %T %Z")
+            }).or_else(|_| {
+                time::strptime(s, "%c")
+                }) {
+                    Ok(t) => Ok(HttpDate(t)),
+                    Err(_) => Err(()),
+                    }
+    }
+}
+
+impl Display for HttpDate {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let tm = self.0;
+        let tm = match tm.tm_utcoff {
+            0 => tm,
+            _ => tm.to_utc(),
+        };
+        fmt::Display::fmt(&tm.rfc822(), f)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use time::Tm;
+    use super::HttpDate;
+
+    const NOV_07: HttpDate = HttpDate(Tm {
+        tm_nsec: 0,
+        tm_sec: 37,
+        tm_min: 48,
+        tm_hour: 8,
+        tm_mday: 7,
+        tm_mon: 10,
+        tm_year: 94,
+        tm_wday: 0,
+        tm_isdst: 0,
+        tm_yday: 0,
+        tm_utcoff: 0,
+    });
+
+    #[test]
+    fn test_imf_fixdate() {
+        assert_eq!("Sun, 07 Nov 1994 08:48:37 GMT".parse(), Ok(NOV_07));
+    }
+
+    #[test]
+    fn test_rfc_850() {
+        assert_eq!("Sunday, 07-Nov-94 08:48:37 GMT".parse(), Ok(NOV_07));
+    }
+
+    #[test]
+    fn test_asctime() {
+        assert_eq!("Sun Nov  7 08:48:37 1994".parse(), Ok(NOV_07));
+    }
+}

src/header/shared/mod.rs

@@ -1,9 +1,11 @@
 pub use self::charset::Charset;
 pub use self::encoding::Encoding;
 pub use self::entity::EntityTag;
+pub use self::httpdate::HttpDate;
 pub use self::quality_item::{Quality, QualityItem, qitem, q};
 
 mod charset;
 mod encoding;
 mod entity;
+mod httpdate;
 mod quality_item;